Event Horizon Telescope

A long standing goal in astrophysics is to directly observe the immediate
environment of a black hole with angular resolution comparable to the event horizon.

Realizing this goal would open a new window on the study of general relativity in the strong
field regime, accretion and outflow processes at the edge of a black hole, the existence of
an event horizon, and fundamental black hole physics. Steady long-term progress on improving
the capability of Very Long Baseline Interferometry (VLBI) at short wavelengths has now made
it extremely likely that this goal will be achieved within the next decade.

The most compelling evidence for this is the recent observation by 1.3 mm VLBI of Schwarzschild
radius scale structure in Sgr A*, the compact source of radio, submillimeter, near infrared and X-rays at the
center of the Milky Way. Sgr A* is thought to mark the position of a ~4 million solar mass
black hole, and because of its proximity and estimated mass presents the largest apparent
event horizon size of any black hole candidate in the Universe.

This new 1.3 mm VLBI detection confirms that short-wavelength VLBI of Sgr A* can and will be used to directly probe the event
horizon of this black hole candidate: in short, Sgr A* is the right object, VLBI is the right
technique, and this decade is the right time.

Over the next decade, our group proposes to combine existing and planned millimeter/submillimeter facilities into a high-sensitivity, high angular resolution
Event Horizon Telescope that will bring us as close to the edge of black hole as we will ever
come. This effort will include development and deployment of submillimeter dual-polarization receivers,
highly stable frequency standards to enable VLBI at 230-450 GHz, higher-bandwidth VLBI backends
and recorders, as well as commissioning of new submillimeter VLBI sites.

We emphasize that while there
is development and procurement involved, the path forward is clear, and recent successful
observations have removed much of the risk that would normally be associated with such an ambitious
project.